The study of the base specificity induced in the mode of action of a number of deoxyribonucleases caused by the reversible modification of DNA with N-cyclohexyl-N'-Beta-(4-methylmorpholinium) ethylcarbodiimide will be continued. The modification of the terminals of DNA molecules and polydeoxyribonucleotides with polyhydroxy "handles" will be carried out in order to provide a method for the isolation of large fragments containing the orginal terminals for subsequent sequence analysis.